Upon contact between two bodies, a polyaxial stress state is formed under the contact surfaces and can be converted into material strain via the equivalent stress or shear stress hypothesis. This is also the case if both bodies roll on one another; by way of example, a bail can roll on a plane or in a ball groove. A material is resistant to rolling if it can withstand this loading durably, without damage arising as a result of material fatigue. An inadequate rolling strength becomes apparent initially in a microstructural transformation or in a progressed stage in destruction of the raceway contour by the breakaway of small material fractions. Depending on the nature of contact (point or linear contact), a hardness of 58 HRc or 650 HV at the site of the greatest pressure is deemed to be sufficient for the required rolling strength.
Heat-treated steels are suitable for hardening with subsequent tempering on account of their carbon content. A heat-treated steel can be an alloyed steel. An alloy is described by indicating the mass fractions of the components thereof as a percentage. The percentages indicated for describing the present invention are mass data which indicate the percentage in relation to the overall weight. The heat-treated steels include steels for surface layer hardening in accordance with DIN 17212. In the case of partial hardening, in the surface layer of the component, only the surface layer has a martensitic microstructure, with the core having a heat-treated microstructure,
Hardening refers to the process of rapid cooling from the austenite area, with the austenite being transformed into martensite.
Tempering is understood to mean heating after preceding hardening to a temperature below “Ac1.” This can be effected with the objective of a heat treatment or for reducing internal stresses in the microstructure. In the iron-carbon diagram, Ac1 denotes the first transformation point upon heating of the steel, This transformation point denotes the start of the alpha-gamma transformation (start of austenite formation).
Bar material denotes, in particular, bars having any desired cross-sectional. profile, for example rectangular profiles and round profiles. The length of the bars can vary depending on the use; lengths of up to several meters are possible.
A known heat-treated steel Cf53 with the material number 1,2113 is normalized or spheroidized for subsequent machining,
The intention of the normalizing is for a uniform microstructure with a fine grain to he formed in the material. It is employed for eliminating non-uniform and coarse microstructures.
In the case of spheroidizing, the focus is on the change in the elongation at break and in the hardness. As a result of the annealing, the strip-like cementite loses strength and can follow up in striving for a body with the smallest possible surface. Granular cementite is formed, which is why this is also referred to as spheroidizing. As a result, it is easier to form and machine the material.
Soft annealing achieves an improvement in machinability in the case of steels having a carbon content of greater than 0.4% by weight, There is a dependency between the mechanical properties and microstructure constituents (ferrite, pearlite) and also the distribution thereof and the degree of spheroidization of the carbides. These annealed steels have a relatively low core strength.
Said heat-treated steels are often pre-heat-treated as round bars. In the case of average and higher diameters, 20 to 40 mm, non-uniform through-hardenability or heat treatability may he present, and consequently inhomogeneous mechanical properties and also residual stresses and distortion may be the result. By way of example, the low-alloyed heat-treated steel Cf53 can only be heat-treated all the way through as bar material to a diameter of less than 20 mm. The bar material which has been heat-treated all the way through has a largely homogeneous heat-treated microstructure, as seen over the cross-section.
The round bars which have been pre-heat-treated can be machined by chip-forming processes or forming processes and finally hardened and tempered. During the hardening, it is often the case that merely the surface layer is hardened.
A defined increase in the mechanical properties which is tolerated within relatively narrow limits can be achieved via heat treatment of the bar cross-section. This is a foundation of controlled cold forming processes. An even better hardenability of the surface layer is possible out of the heat-treated microstructure.
The hardness penetration is an important criterion for a high-quality heat treatment. The objective of a heat treatment within the context of the abovementioned application is to produce uniform mechanical properties.